Kate Becker: Searching for flaws in Einstein's theory of gravity

Headlines are forever boasting that the latest study will expose Einstein's mistakes. The world's most precise clock could prove Einstein wrong. 12-year-old genius thinks he can prove relativity wrong. Neutrinos prove Einstein wrong. Only rarely do you read New study says Einstein was exactly right!

So when I read a press release headlined "Newly discovered three-star system could debunk Einstein's theory of General Relativity," as if the theory of gravity were just another myth to be busted, I thought: Poor Einstein. Why all the hate?

The new system is made up of three dead stars: a pulsar and two white dwarfs. The pulsar, an extremely dense neutron star that spins once every 2.7 milliseconds, was discovered at the Green Bank Telescope in West Virginia. Astronomers then found it was orbiting a somewhat less dense stellar corpse called a white dwarf, and finally that the pulsar and white dwarf were orbiting a third object: another white dwarf. The orbits inscribed by the trio would fit within the Earth's orbit around the Sun.

There are plenty of "three-body" systems in the Milky Way. What makes this one so special? First of all, the pulsar. Pulsars are natural laboratories for testing General Relativity. That's because pulsars emit beams of radio waves from near their magnetic poles. As a pulsar spins, those radio beams sweep through space and, if everything lines up right, can be picked up by radio telescopes here on Earth.

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Every time the pulsar spins, we see another radio pulse--in the case of this pulsar, that's 366 pulses in a single second.

Pulsars "tick" so regularly that some rival the best atomic clocks on Earth, and by looking for miniscule inconsistencies in this ticking, astronomers can begin to map out the conditions around the pulsar. Which brings us to the second special thing about this system: Its second and third members are white dwarfs, which are close and massive enough to set up strong gravitational interactions with the pulsar. This gives astronomers an opportunity to test an essential piece of General Relativity called the strong equivalence principle, a variation of the familiar adage that if you drop a bowling ball and a feather on the moon, they'll both hit ground at the same moment. In other words, an object's acceleration due to gravity doesn't depend on its mass or what it's made of. The strong equivalence principle extends this rule to extremely dense objects, like pulsars, arguing that objects with strong "self-gravity" --essentially, objects held together by lots of gravitational energy--should feel the same gravitational acceleration as those with weaker self-gravity.

You can imagine the pulsar and its orbiting white dwarf as the bowling ball and the feather, both falling toward the second white dwarf. Will the pulsar, which has very strong self-gravity, fall in just the same way as the white dwarf, which has weaker self-gravity?

That's what astrophysicists now hope to find out. Which brings us back to our original question: Why go to such lengths to prove poor Einstein wrong?

Theorists have good reason to believe that General Relativity must break down at some point. It fundamentally clashes with that other great theory of physics, quantum mechanics, and novel theories of gravity predict that we should eventually see cracks in the strong equivalence principle. But, so far, both quantum mechanics and Einstein's theory of gravity have passed every test physicists and astronomers have dreamed up. They are frustratingly impervious, the New York Yankees of physics.

Yet physics only advances when we can discover how existing theories are imperfect and incomplete. In other words: It's nothing personal, Einstein. In physics, "debunking" may be the sincerest form of flattery.

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